The following account of the background relates to one of the areas of application of the present application, hearing aids for electrical stimulation of the cochlear nerve, typically termed ‘cochlear implant hearing (assistance) devices’ or simply ‘cochlear implants’ (CI).
Cochlear implant hearing assistance devices have been known in many years in a variety of configurations, but typically comprising    a) a number of electrodes implantable in different locations of the cochlea allowing a stimulation of different frequencies of the audible range,    b) an external part for picking up and processing sound from the environment, and for determining sequences of pulses for stimulation of the electrodes in dependence on the current input sound,    c) a (typically wireless, e.g. inductive) communication link for simultaneously transmitting information about the stimulation sequences and for transferring energy to    d) an implanted part allowing the stimulation to be generated and applied to the relevant of said electrodes.
Such systems are e.g. described in U.S. Pat. No. 4,207,441 and in U.S. Pat. No. 4,532,930.
A cochlear implant electrically stimulates the auditory nerve of a deaf patient to produce a sound perception. The CI-devices typically have between 12 and 24 processing channels which encode the sound energy level at different cochlear locations. Due to electrical current spread within the scala tympani (i.e. the cochlear duct in which the electrode array lies), the spectral resolution is reduced and therefore the speech perception performance of cochlear implant patients does not improve when the number of activated channels is increased higher than appr. 8 to 12. This is in contrast to normally hearing listeners which benefit from an increasing number of processing channels in psycho-acoustic studies using speech vocoders.
The problem is well-known and different solutions have been envisioned to solve it. Most of the work has been concerned with focusing current using multi-polar stimulation. This current focussing technic relies on a general principle of beam-forming using multiple sources. Beam-forming is used in various technologies such as radar or microphone array. The results of current focussing have been disappointing for different reasons. First, power consumption grows linearly with the number of channels used. Second focusing the electric field can have subtle sub-threshold effects from the side-lobe producing the focus. Because of these drawbacks, other works to reduce the spread of excitation are on-going. Recently another approach has been to use optical stimulation produced by laser pulses in the infra-red range. This new stimulation technique may not have the same level of spreading but is still currently very power consuming.